Qualifications
Same as almost every other career in the medical profession—four years of undergrad, four years of medical school, and then at least three years of residency. And that doesn't mean proving that you've lived in the state of California for the past three years. You'll be almost thirty before you make your first dollar, and you'll likely have a bunch of debt (source).
Also, as you're dealing with a lot of families, it's especially important to have a heart of gold. Just knowing your stuff is not enough—you have to make people feel better about their physical condition, even when you know there isn't much to feel better about. In that sense, you're almost like a paid actor, which might be a difficult concept to wrap your head around.